World's weirdest slug is shaped like a fish and glows in the dark

Phylliroe is a type of nudibranch, or sea slug, that has evolved to look and swim like a fish.

This sea slug is shaped like a fish in a bizarre example of convergent evolution.
(Photo: Alain Diringer via seaslugs.free.fr)

If you saw this bizarre creature swimming around in the ocean, you'd be forgiven for mistaking it for a fish. Although it's about the size of a goldfish and has a body shaped like a fish — including what looks like a caudal fin and even rudimentary dorsal, pelvic and anal fins — this is no fish. It's actually a sea slug, or more precisely, a nudibranch of the genus Phylliroe, reports Deep Sea News.

Phylliroe's fish-like appearance is a stellar example of convergent evolution. Essentially, this basic body shape is particularly good for swimming in open ocean. So, at some point, its ancestors abandoned their sluggardly walk along the sea floor and took to swimming instead, and thus evolved a body design better suited for their new lifestyle.

Its shape and movement aren't the only things about Phylliroe that are fish-like. It also hunts kind of like a fish, known to swim and stalk its favorite prey — jellies — from below. It's a fierce predator, and surprisingly quick in the water. Phylliroe also sports some pretty mean-looking horns, called rhinophores, which are actually sense organs that it uses to sniff out its quarry.

As if Phylliroe wasn't eccentric enough, it's also transparent. You can actually see its internal organs churning away inside of it. Oh, and it glows. That's right, Phylliroe is a rare example of a bioluminescent sea slug, capable of producing its own light. It's like a little swimming sea lantern, undoubtedly a spectacular sight for anyone who has witnessed it in nature.

Phylliroe's juveniles are known to be parasitic, attaching themselves to the bell of the hydromedusa Zanclea costata, slowly consuming the jelly until becoming large enough to swim on their own. In other words, this nudibranch might be a jelly's worst nightmare.

As if it wasn't already weird enough, Phylliroe also poops from its side. So there's also that.